Two radio frame structures are supported in LTE. The first radio frame structure is applicable to Frequency Division Duplex (FDD), which is a mode of bidirectional communication in which transmission and reception take place at the same time on different carrier frequencies. The second radio frame structure is applicable to Time Division Duplex (TDD), which is a mode of bidirectional communication in which transmissions in each direction take place on the same carrier frequency but in different time slots.
In both frame structure types, a radio frame of 10 ms is divided into two half-frames of 5 ms, with each half-frame consisting of five subframes of length 1 ms. In the TDD frame structure, each subframe is either a downlink subframe, an uplink subframe or a special subframe. LTE TDD supports the various uplink-downlink configurations provided in FIG. 1. With reference to FIG. 1, for each subframe in a radio frame, “D” denotes that the subframe is reserved for downlink transmissions, “U” denotes that the subframe is reserved for uplink transmissions and “S” denotes a special subframe with the three fields. The first field corresponds to a Downlink Pilot TimeSlot (DwPTS), which is a special downlink timeslot occurring in the 2nd subframe (and in some configurations the 7th subframe) of each radio frame. The length of the DwPTS is variable to allow for different DownLink (DL)-UpLink (UL) switching periods to be configured. The second field corresponds to a Guard Period (GP), which is a time period used to prevent overlap between two different signals. The GP can consist of a transmission gap or be filled with a signal whose correct reception is not essential. The GP can be used to prevent overlap between transmission and reception. The third field corresponds to an Uplink Pilot TimeSlot (UpPTS), which is a special uplink timeslot occurring in the 2nd subframe (and in some configurations the 7th subframe) of each radio frame. The UpPTS has a length of 1 or 2 Single Carrier Frequency Division Multiple Access (SC-FDMA) symbols and is used only for transmission of a shortened Random Access Channel (RACH) preamble or Sounding Reference Signal (SRS).
Suitable lengths for DwPTS and UpPTS are identified in FIG. 2 and are subject to the total length of DwPTS, GP, and UpPTS being equal to 1 ms. Each subframe consists of two slots of length 0.5 ms. Uplink-downlink configurations with both 5 ms and 10 ms downlink-to-uplink switch-point periodicity are supported. In case of 5 ms downlink-to-uplink switch-point periodicity, the special subframe exists in both half-frames.
In case of 10 ms downlink-to-uplink switch-point periodicity, the special subframe exists in the first half-frame only. Subframes 0 and 5 and DwPTS are always reserved for downlink transmission. UpPTS and the subframe immediately following the special subframe are always reserved for uplink transmission.
In a TDD cell, a TDD configuration is characterized by both uplink-downlink configuration and special subframe configuration. Therefore the term TDD configuration used hereinafter refers to a combination of uplink-downlink configuration (e.g. one of the configurations in FIG. 1) and a special subframe configuration (e.g. one of the configurations in FIG. 2) configured in a TDD cell. It is appreciated that more TDD configurations may be introduced in future. It is further appreciated that the present application is not intended to be limited to the existing TDD configurations and may be applicable to additional or alternative supported configurations.
Additional information relating to TDD configuration signaling and management in LTE networks is provided in 3GPP RP-121772, “Further Enhancements to LTE TDD for DL-UL Interference Management and Traffic Adaptation,” as well as 3GPP TS 36.211, v11.3.0, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation,” the disclosures of which are incorporated by reference in their entirety.